1. Field of the Invention
The present invention relates to photographic apparatus, in general, and more particularly to a mechanism for urging a pressure member against a photographic film to locate an exposure area of the film in an exposure plane.
2. Description of the Prior Art
It is well known in still picture cameras that when a film frame is moved during exposure or does not lie substantially flat in the focal plane of the picture-taking lens, the resulting picture will be blurred or distorted. For this reason, there is usually provided a pressure-applying assembly for securely holding the film frame in the focal plane of the lens during exposure. Such pressure-applying assemblies are known in a wide variety of constructions. However, the known constructions suffer from several disadvantages. For example, many pressure plates are mounted on a loading door or a removable rear wall of the camera, the pressure plate being urged by a compression or leaf spring to bear against the film, as in U.S. Pat. Nos. 3,311,037 and 3,707,904 and in the above-referenced Harvey patent applications (which are generally described in Publication No. 172,108 of Research Disclosure, Aug. 1978, Number 172.) Because in most constructions, the pressure plate and the spring protrude from the loading door, they are accessible to damage or deforming during the time the loading door is opened. Moreover, such an arrangement usually adds to the front-to-back dimension of the camera, which makes it difficult to design a camera that is relatively thin, such as a pocket-size or miniature camera.
Another type of pressure-applying assembly, as in U.S. Pat. No. 1,829,332, includes a cam mechanism which is actuated by movement of a shutter operating mechanism to hold a pressure plate firmly against the film. Here, the cam urging of the pressure plate is not resilient, as in the example of the spring-urged pressure plate, and may require substantially more front-to-back spacing in the camera than the spring-operated version.
A number of spring-operated and cam-operated pressure-applying assemblies have been devised in which a pressure member is relievable with respect to the film, as in the above-referenced Harvey patent applications and U.S. Pat. No. 1,829,332. That is, the pressure member is retracted from the filmstrip after exposure to permit film advance. Such relievable pressure-applying assemblies, in addition to suffering from the problems described above, are relatively more complex and expensive than the non-relievable types.